Well completion and logging method



mp -n11 m 3919312105 July 6, 1965 3,193,005

F. H- HUNTER ETAL WELL COMPLETION AND LOGGING METHOD Filed Oct. 24, 1961 g 3 Sheets-Sheet 1 FIG. I

FRANK H. HUNTER GEORGE W. WIDNER INVENTORS al ivfg v AGENT y 6, 1965 F. H. HUNTER ETAL 3,193,005

WELL COMPLETION AND LOGGING METHOD Filed Oct. 24, 1961 a Sheets-Sheet 2 FRANK H. HUNTER GEORGE W. WIDNER INVENTORS AGENT July'6, 1965 F.'H- HUNTER ETAL 3,193,005

WELL COMPLETION AND LOGGING METI- IOD Filed 001:. 24, '1961 3 SheetsSheet 3 INDUCTION LOGS CONFORMANCE TEST 4| OPEN HOLE PLASTIC CASED HOLE RESISTIVITY OHM-METERS GEORGE W. WIDNER INVENTORS AGENT United States Patent WELL COMPLETION AND LOGGING METHOD Frank H. Hunter and George W. Widner, Tulsa, Okla.,

assignors, by mesne assignments, to Esso Production Research Company, Houston, Tex., a corporation of Delaware Filed Oct. 24, 1961, Ser. No. 147,326 6 Claims. (Cl. 166-4) This invention is directed to a method for increasing the efficiency of petroleum recovery from subterranean reservoirs. Broadly, the invention is concerned with the problem of Water encroachment which frequently interferes with the production of petroleum from a horizon having a water-oil contact. More particularly, the invention is directed to a method for employing electromagnetic induction logging to follow the progress of water invasion and to determine the displacement efiiciency of a water drive. In its more specific aspects, the invention involves the control of oil flow rate from a well in response to a determination of the migratory behavior of water within the reservoir.

It frequently happens that a commercially attractive subterranean reservoir of petroleum is found in contact with a large aquifer. The water may lie in the same earth stratum or it may lie in an adjacent stratum. Due to the difference in density as well as the immiscibility of these fluids the Water will usually be found immediately below the oil reservoir. Because of natural reservoir pressure, a withdrawal of oil from such reservoirs usually results in a migration of the water into the pores of the formations from which the oil is withdrawn.

This water intrusion has both desirable and undesirable effects. It is undesirable because it usually interferes with the production of oil by entering the producing string of the well, thereby increasing the expense of handling the petroleum; and will ultimately force abandonment of the Well. However, if this migration is uniform or can be controlled it will have the desirable effect of displacing the oil from the porous formation, thereby increasing the efficiency of petroleum recovery without fluid injection. Particularly desirable, is a reservoir wherein the water assists in displacing the oil by exerting a driving force which actually pushes the oil to the well bore and thence to the surface of the earth. Such a reservoir is known as a water drive reservoir, which results either from expansion of the water phase in some instances, or from hydrostatic head where the body of water extends to a higher level in some adjoining area.

In the completion of an oil well wherein the case well bore penetrates an entire oil-bearing formation, superficial analysis indicates that it should be desirable to perforate the casing substantially throughout the pay zone. However, in many existing oil fields, through fear of water coning, it has been conventional practice to perforate opposite only the upper portion of the pay zone. Since the production rate increases as a greater fraction of the pay zone is perforated, this practice has been a deliberate sacrifice of oil productivity in order to guard against the possible intrusion of water, without any real knowledge as to whether water coning would or would not result. Now, in accordance with this invention, a method is provided for determining the characteristic behavior of a water-oil contact within a given oilbearing horizon, such that wells may be completed in the area with perforations extending as low as possible into the pay zone without producing water.

The rate at which oil is produced from a water drive reservoir is very critical in determining the efficiency of oil displacement. If the oil is produced at too rapid a rate it usually causes a coning effect or a fingering of 3,193,005 Patented July 6, 1965 the water into the producing string, by-passing large quan tities of oil. Once water has entered the producing string, no simple remedy is known for correcting this condition. That is, oil does not readily re-enter the area where water has once encroached. Therefore, the perforations opposite the affected area are usually cemented off, which is an expensive, time-consuming operation. It is one object of this invention to provide a practical means of following the behavior of water migration in such a reservoir in order to optimize the efficiency and rate of oil recovery. Thus it is desired to produce the oil at a controlled rate such that the water level throughout the reservoir rises evenly and such that water coning is stabilized at a height safely below the level of well perforations.

In accordance with this invention a method is provided whereby electromagnetic induction logging may be employed to measure changes in water saturation resulting from migration in the reservoir. In conventional practice induction logging is used only in uncased well bores. In an uncased well bore, satisfactory study of water migration cannot be accomplished because the fluids in the well bore may resaturate the formation and thereby prevent the log from showing true reservoir conditions. Conventional electrical logging, including induction logging, will not work in boreholes cased with metallic pipe. This problem is overcome in accordance with the present invention by employing a casing which includes a section of non-conducting pipe opposite the region under study.

In accordance with one embodiment of the invention a well bore of relatively larger diameter is drilled in a conventional manner to the top of the pay zone. Ordinary steel casing is set to this level by cementing in the usual manner. Drilling of a smaller bore is then resumed through the pay zone. Open hole logging is employed to detect the water-oil contact and the well is bottomed at least 50 feet below the water-oil contact. It is helpful to core the entire pay zone. Routine core analysis should include vertical and horizontal permeability and porosity for each foot of core, and vertical permeability of the longer core sections. An induction log in the open hole is useful in interpreting subsequent logs run in a fully cased hole. The remaining portion of the hole is then cased with a non-conducting plastic liner to permit the running of in duction logs, unaffected by well-bore invasion, throughout the interval under study. A well completed in this manner without perforating the plastic liner may be used as an observation well only. In a preferred embodiment of the invention, however, the well is used concurrently as a production and observation well by perforating the plastic liner in the upper portion of the pay interval so that the induction log may be run below the level of the perforations in order to follow the movement of the wateroil contact. Ideally, the water-oil contact should rise at substantially the same rate throughout the pay interval. Extremely low production flow rates will obviously accomplish this result; however, economic considerations dictate that the oil must be removed at a relatively high flow rate. Thus, coning to a limited extent is tolerated as a compromise.

In accordance with a preferred embodiment of the invention a single well completed in the above manner is perforated in the upper portion of the pay zone, for example, along the top 25 percent of the oil zone. The well is then produced at a rate equal to or greater than its anticipated on-stream rate. When the producing rate is stabilized the pressure draw-down and flow rate are carefully measured and the total volume of oil produced is recorded. The theoretical time required for a cone to rise a substantial distance, for example 10 feet, is then calculated assuming the horizontal to vertical permeability ratio suggested by core analysis. A suitable method of calculating the time appears in Petroleum Technology, Vol. 9, No. 5, pp. 81-111, September 1946. After flowing the well at a constant draw-down pressure differential for a period of time indicated by the theoretical calculation, the well is logged while flowing, using the magnetic induction technique to detect the actual cone height along the unperforated portion of the plastic liner. After a cone height versus time relationship has been established for the initial flow rate, the well is shut in and an induction log run every third day to determine the rate at which the cone recedes. After the rate of cone collapse is established, two additional production tests are run at rates 25 to 30 percent above and 25 to 30 percent below the initial test rate. This establishes a relationship between production rate and coning height for this well, and also allows calculation of an effective horizontal to vertical permeability ratio for the formation. This information is then utilized in the completion of all wells in the area, to establish an optimum width of the perforation band, such that production flow rates are controlled to stabilize cone heights a safe distance below the lowest perforations. Producing wells in the test area completed prior to the coning study of the invention are (reopened and also perforated a greater distance into the pay zone, if the results obtained from the test well indicate that a significant increase in productivity can be thereby achieved without danger of producing water.

For example, if a pressure differential of 500 p.s.i. is found to cause a water cone height of 20 feet, and the oil-bearing horizon is 70 feet thick, then most of the remaining 50 feet is considered available for well completions in the given area. A substantial distance will be left between the lowest perforations and the expected maximum coning height, of course, in order to allow not only for local variations in horizontal to vertical permeability ratios, but also to allow for a gradual rise of the oil-water contact through the area. Thus, perforations should extend no more than 40 feet into the oil zone of the example test area.

Alternatively, coning height is controlled by adjustment of the draw-down pressure differential. If existing wells in the example test area have been perforated only 20 feet into the oil zone, a logging test is made to determine the maximum permissible draw-down in order to stabilize coning approximately feet below the lowest perforations. Over a long period of time, as the oil-water contact rises throughout the area, the draw-down differential is gradually reduced to keep the cone from rising into the production level.

In addition to determining the optimum production rate from a water-drive reservoir, induction logging data obtained in the manner described provides an excellent basis for determining the displacement efliciency by water-drive. That is, the porosity of the pay zone is determined by coring or other accepted methods in order to calculate water saturation as a fraction of pore volume. The knowledge of how much oil is left in the formation is important in order to determine the practicality of secondary recovery operations.

FIGURE 1 shows the simplest example of a productionobservation well completed with a plastic liner section in accordance with the method of this invention.

FIGURE 2 shows the preferred embodiment of this invention wherein a larger hole is drilled to the top of the producing zone and an ordinary steel casing string is cemented in the usual manner to this level. Then, a smaller bore is drilled through the producing horizon, and lined with plastic pipe.

FIGURE 3 shows a graphic comparison of an induction log run in an open hole, with an induction log run in the same hole lined with plastic pipe.

Referring now to FIGURE 1, the well bore is drilled to and through the producing zone with substantially the same uniform diameter. The plastic pipe section carrying a conventional float shoe is attached to the end of an ordinary steel casing string which is then pushed into the hole. The well is completed in a producing horizon characterized by an oil-bearing zone 11 and a waterbearing zone 12 separated by the original contact interface 13. The well bore is bottomed a substantial distance into the water zone for convenience. The first section of casing run into the well bore is the plastic liner section 14. The remainder of the casing is ordinary steel pipe 15. At the bottom of the well bore is conventional float shoe 16 used in cementing the liner, form ing cement zone 17. Although the well may be used as an observation well solely, in which case no perforations will be needed, the embodiment shown includes perforations 18 in the upper portion of the oil-bearing strata. The liner opposite only the upper portion of the oil layer is perforated in order to leave ample space for following the rise of water cone 20 with induction logging tool 19 in the remaining portion of the plastic liner section. This arrangement enables the well to be used as a productionobservation well.

Referring to FIGURE 2, the preferred completion procedure for carrying out this invention is described. In this procedure a relatively larger well bore is drilled to the top of the producing horizon 11. Again the producing horizon is characterized by a water-containing stratum 12 and original water-oil contact 13. After the steel casing 15 has been set to the top of the pay zone in a conventional manner, the entire pay interval is cored with a small diameter bore, and the well is bottomed at least 50 feet below the water-oil contact. The core is carefully examined for dense streaks that may be vertical permeability barriers and these streaks are checked for permeability. Routine core analysis should include vertical and horizontal permeability and porosity for each foot of core and vertical permeability of the longer core sections.

Installing the plastic pipe liner consists of four major operations. One, making up the plastic liner and liner hanger assembly; two, running the tubing into the liner and attaching it to the liner shoe; three, running tubing and liner into the hole, and four, cementing the liner. These operations are described step by step below in order to present a clear picture of the preferred operation in accordance with the method of this invention.

A conventional cement float shoe 16 is attached to the bottom of the first joint of plastic liner 14. The length of plastic liner is preferably selected to extend at least 200 feet up into the casing. Conventional casing slips are used, but in handling the plastic pipe only about twothirds of the torque applied to steel casing should be tolerated in handling the pipe. Rubber centralizers are installed in alternate joints of the pipe using non-metallic bands.

Tubing '22 is then run through the liner and attached to the liner shoe. The first joint of tubing has a connection 23 for fitting into the liner float shoe. Tubing is run into the liner until this fitting seats on the shoulder in the plug receptable in the float shoe. The liner hanger 24 and tubing packer 25 are optional; however, they are desirable in order to provide greater stability in the assembly. Centralizers 26 and 27 are also desirable to ensure best results.

The liner assembly is then run in the hole to the bottom with the tubing. This step is carried out very carefully and slowly to prevent sudden shock loads on the plastic liner. It is helpful to keep the liner full of mud as it is run into the hole to aid in preventing collapse.

This plastic liner setting assembly and procedure provides the following features. Tubing thrust while the liner is being run into the hole is transmitted to the liner shoe. This keeps the liner in tension as it passes tight spots in the hole and prevents buckling stresses in the plastic pipe. A spear located in the tubing string near the top of the liner will keep the plastic pipe in tension if upward pull is necessary to free a stuck liner. The liner hanger is used to prevent any back pressure likely to cause lost circulation. A seal cap at the top of the liner prevents excess cement pumped up into the casing above the liner from falling into the liner around the tubing. As soon as the cement is placed the tubing can be raised to wash out excess cement above the liner.

The final step in this completion procedure is the cementing of the liner assembly. To prevent loss of circulation while the cement is being placed, a low weight high permeability slurry containing a bridging additive is desirable. Because somewhat brackish field water may be used for mixing, the cement selected must not be affected by salt or sulfate content. After the liner assembly is hung, circulation is established for a period long enough to ensure that circulating pressure is at a minimum and no circulation is being lost. Circulation is maintained during the cement mixing operation. The calculated amount of cement necessary to complete the operation is pumped into the tubing and followed with a conventional plug. The plug is pumped down with mud or water until the plug stops in the liner shoe receptacle.

Possible collapse of the plastic liner is an important consideration in conducting this cementing operation. Assuming that the liner is filled With water, a cement slurry behind the liner will exert a static collapsing pressure of approximately 200 p.s.i. This is well within our laboratory collapse test rating of a preferred pipe, but surge pressure should be guarded against. After the cement is in place, the tubing is lifted far enough to move the releasing spear and seal cap out of the liner hanger. Circulation is maintained to wash excess cement from above the liner hanger and the tubing is then lifted and removed from the hole.

The well should then remain shut in for several days during which time the induction log is run at least twice to make certain that reproducible logs are being obtained. The apparent formation resistivity may change slightly with time as mud and cement filtrate dissipates and it Will be necessary to determine if these changes are significant.

The well completed in this manner may be used as an observation well solely, in which case no perforations will be made. In accordance with the preferred embodiment of the invention, however, the well is used concurrently as a production-observation Well. According, approximately the upper one-fourth of the liner opposite the oil zone is provided with perforations d8. Logging tool 19 is positioned within the lower portion of the liner, to follow the rise of water cone 20.

The completion procedure of FIGURE 2 is preferred over that of FIGURE 1 because it eliminates the problem of running the plastic pipe through great lengths of open hole. That is, the plastic liner is not placed in compression that may cause buckling stresses if it is run through a tight or crooked hole. Moreover, when the liner is cemented below a larger casing, it can be readily drilled out and replaced.

Referring to FIGURE 3, a graphic comparison is presented, of results obtained from a logging test which demonstrates that the plastic liner does not interfere with induction logging. That is, a given interval of an uncased borehole was induction logged, fol-lowed by completion of the well by installing the plastic liner, and then the same interval was again logged. The logs are substantially the same.

The section of casing which is used to line the oilproducing horizon may be any non-conductor of electricity, provided the material has the necessary structural strength to resist the normal stresses to which a casing material is subjected. Suitable plastics include poly ethylene, polyvinyl chloride, and polyester resins. A preferred liner material is Fiberglas-reinforced epoxy resin pipe. The preferred pipe is manufactured by tightly wrapping individual threads of Fiberglas in parallel on a mandrel in a spiral. A set of Fiberglas threads forming a strip 2 inches wide is passed through an epoxy resin bath and then onto the mandrel under tension. At the end of the mandrel the direction of the spiral is reversed. The result is a criss-cross structure resembling the cross lacing of the steel bands visible on the outside of a highpressure grease-gun hose. The close spacing of the threads and the tension produce adhesion between the threads with a maximum of Fiberglas and a minimum of resin. After the desired wall thickness has been wrapped, the pipe and mandrel are placed in a curing oven and the pipe cured with tension in the threads. The mandrel is then withdrawn and the pipe is complete. Pipe manufactured in accordance with this procedure may be obtained from Amercoat Corporation of South Gate, California.

Electromagnetic induction logging techniques and equipment are well known in the petroleum industry and need not be particularly described for the purposes of this disclosure. In this regard, further information may be obtained by reference to an article by H. G. Doll, Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil Base Mud, Journal of Petroleum Technology, Vol. 1, No. 6, June 1949, page 148 ff.

The foregoing description and illustrations have been by way of example only and are not intended to limit the invention. Instead, the scope of the invention is defined by the following claims.

What is claimed is:

1. In the recovery of petroleum from an oil bearing horizon penetrated by a well bore, said horizon having an oil-water contact, a test procedure for determining the relationship between water coning height and draw-down pres sure differential which comprises casing said bore opposite said horizon with a non-conductor of electricity, perforating said casing at a level of said horizon substantially above said oil-water contact, flowing said well at a substantially constant drawn-down pressure differential, and logging said horizon by electromagnetic induction while flowing said well, to d e t ern 1 ine the height of vaggjntrusioncausedby said pressure diiferential.

2. A method for increasing the efliciency of petroleum recovery from an oil-bearing horizon penetrated by a well bore, said horizon having an oil-water contact, which comprises casing said bore opposite said horizon with a nonconductor of electricity, perforating said casing at a level opposite said horizon substantially above said oil-water contact, flowing said well and logging said horizon by electromagnetic induction to determine the rate of water encroachment and limiting the rate of oil production from said horizon to a value not greater than that which corresponds to maximum permissible encroachment, as indicated by said log.

3. A method for increasing the efliciency of petroleum recovery from an oil-bearing horizon penetrated by a plurality of producing wells, said horizon having oil-water contact, which comprises drilling a test well bore through said horizon, casing said test well bore opposite said horizon with a non-conductor of electricity, perforating said casing at a level opposite said horizon substantially above said oil-water contact, flowing said test well, logging said horizon by electromagnetic induction while flowing said test well to determine the height of water coning and limiting the rate of oil production from at least one of said producing Wells to a value not greater than that which corresponds to maximum permissible coning, as indicated by said logging data.

4. A method for increasing the efliciency of petroleum recovery from an oil-bearing horizon penetrated by a producing oilwell, and having an oil-Water contact, which comprises drilling a test wellbore through said horizon, casing said test wellbore opposite said horizon with a nonconductor of electricity, perforating said casing at a level substantially above said oil-water contact, flowing said test well, logging said horizon by electromagnetic induction While flowing said test well, to determine the height of water coning, and extending the perforation band of said producing oilwell to a level safely above the height of said water coning.

5. In the recovery of petroleum from an oil-bearing horizon penetrated by a wellbore, said horizon having an oil-water contact, a test procedure for determining the relationship between water coning height and draw-down pressure differential, which comprises casing said bore opposite said horizon with a non-conductor of electricity, perforating said casing only at a level of said horizon substantially above said oil-water contact, producing said well at a first substantially constant draw-down pressure differential, periodically logging said horizon by electromagnetic induction while producing said well, until the oilwater contact becomes stabilized at a coning height which corresponds to sad first draw-down pressure differential, then changing the flow rate at said well by imposing a second substantially constant draw-down pressure differential thereon, periodically logging said horizon by electromagnetic induction while producing said well at said second draw-down pressure differential, whereby a relationship is determined between water coning height and draw-down pressure differential.

6. A method of increasing the efficiency of petroleum recovery from an oil-bearing horizon penetrated by a producing oil well, and having an oil-water contact, which comprises drilling a test wellbore through said horizon, casing said test wellbore opposite said horizon with a nonconductor of electricity, perforating said casing only at a level substantially above said oil-water contact, logging said horizon by electromagnetic induction while producing said test well through said perforations to determine the maximum draw-down pressure differential which may be maintained without excessive water coning, and then increasing the draw-down pressure differential maintained at said producing oil well to the maximum value which is consistent with a safe water coning height, as determined by said logging.

References Cited by the Examiner UNITED STATES PATENTS 2,258,616 10/41 Kendrick 166--4 3,055,424 9/62 Allen 166-46 3,057,409 10/ 62 Grossman 166242 CHARLES E. OCONNELL, Primary Examiner. BENJAMIN BENDETT, Examiner. 

1. IN THE RECOVERY OF PETROLEUM FROM AN OIL BEARING HORIZON PENETRATED BY A WELL BORE, SAID HORIZON HAVING AN OIL-WATER CONTACT, A TEST PROCEDURE FOR DETERMINING THE RELATIONSHIP BETWEEN WATER CONING HEIGHT AND DRAW-DOWN PRESSURE DIFFERENTIAL WHICH COMPRISES CASING SAID BORE OPPOSITE SAID HORIZON WITH A NON-CONDUCTOR OF ELECTRICITY, PERFORATING SAID CASING AT A LEVEL OF SAID HORIZON SUBSTANTIALLY ABOVE SAID OIL-WATER CONTACT, FLOWING SAID WELL AT A SUBSTANTIALLY CONSTANT DRAWN-DOWN PRESSURE DIFFERENTIAL, AND LOGGING SAID HORIZON BY ELECTROMAGNETIC INDUCTION WHILE FLOWING SAID WELL, TO DETERMINE THE HEIGHT OF WATER INTRUSION CAUSED BY SAID PRESSURE DIFFERENTIAL. 